The preparation of electrical insulation plastic foams using sulfur hexafluoride adsorbed on molecular sieves is described in Canadian Pat. No. 880,377. The method therein described involves mixing of a hardenable plastic forming material and a molecular sieve having sulfur hexafluoride adsorbed therein, followed by heating the mixture to a temperature sufficient to cause the hardenable plastic forming material to harden to a solid plastic and to cause the sulfur hexafluoride to desorb from the molecular sieve to form a cellular structure in the solid plastic. In order to obtain a foam having a substantially uniform cellular structure and a void-free surface or skin, it was necessary to prepare the sieve-plastic mixture by the use of high shear mixing conditions. The use of low shear mixing or less intensive mixing conditions resulted in an inadequate distribution of the sieve within the plastic material which, in turn, resulted in a non-uniform cellular structure and large surface bubbles when the plastic sieve mixture was heated to form the foamed material.
The high shear mixing technique generates a substantial amount of heat causing the sulfur hexafluoride to desorb the sieve prematurely. It is therefore necessary to either very carefully effect the mixing or to provide cooling to the materials as they are being mixed. Even with careful attention to the mixing procedure, some premature desorption and blowing of the foam occurs and the resulting material often has poor surface characteristics caused by large surface bubbles.
Accordingly, it is the object of this invention to provide a new method by which an electrical insulation foam can be prepared from a hardenable plastic forming material and a molecular sieve having sulfur hexafluoride adsorbed therein which overcomes the foregoing problems. This and other objects of the invention will become apparent to those skilled in the art from the following detailed disclosure.